Undersea explorer Barry Clifford is diving back into the water after he and his team uncovered an unexplored trove of buried treasure that he said may lead to the discovery of more than 400,000 gold coins.

Clifford and his team located a sunken pirate ship, Whydah, off the coast of Cape Cod nearly 30 years ago. The Whydah was a slave ship that was captured by notorious pirate captain "Black Sam" Bellamy in February 1717, just two months before it sank on April 26, 1717.

Since discovering the Whydah -- the only documented pirate ship -- in the 1980s, Clifford and his team have worked to bring thousands of treasures discovered on the ship to the surface, he told ABCNews.com.

"There's just a tremendous amount of material on board the vessel," he said.

Clifford and his team focused on excavating a new site beneath the water's depths this summer, and discovered coins stacked up "like poker chips," sedimentary rocks known as concretions and thousands of lead musket balls buried 20 feet beneath the sand, he said.

"We were getting very exciting about this, finding all of this very heavy material like lead and fragments of gold," Clifford said.

So they returned to the site, just 1,500 feet from the shore, for a three-day expedition to see what lay beneath the sand, which culminated Sept. 1.

"We were all tired, but I wanted to dig one more pit before heading back," Clifford said of the last day of the dig. "There was ink-black fog; you virtually couldn't see the bow of the boat. But we dug a hole and sent a diver down, and he came up within 20 minutes, which is half the time I would have expected him to be back."

The diver's short sojourn into the pit, which Clifford said measures approximately 10 to 15 feet in diameter and is 35 feet below the water's surface, was fruitful, he said.

"[The diver's] bag was so heavy with artifacts, coins and lead shot, that he couldn't lift anymore," Clifford said. "He only excavated a 2-to-3-square-foot area of the pit. He didn't even finish that. The pit was filled with heavy metal."

Among the discoveries was a concretion that measured 18 inches long, which often goes unnoticed by the untrained eye, Clifford said.

The team X-rayed the rock, which looked like it was just a piece of rusted metal, and saw it was stacked inside the sediment with coins and gold, Clifford said.

Immediately, he knew they had come upon a discovery he had learned about through a colonial-era document obtained by the project's now-deceased historian, Ken Kinkor.

The document details that before the Whydah went down, Bellamy had stored treasures he had stolen from two other ships on board, including more than 400,000 one-ounce gold coins from the two vessels combined, Clifford said.

"Bellamy put all the treasure aboard the Whydah," he said. "We assume that there was a tremendous amount of treasure on board."

But Bellamy captured 50 other ships during his reign, and "one of the other ships had 4 1/2 tons of treasure," Clifford said.

"That's one of the things we've been looking for: 400,000 gold coins is something we just learned in addition to that. It's crazy," he said.

With the latest discovery, Clifford and his team are planning to head back down into the pit Tuesday or Wednesday to begin what likely will be a three-day effort to unearth more of the pirate plunder, he said.

"This is like a Hansel and Gretel thing: We're following the clues; we're following the breadcrumbs," he said. "But in this case it's gold doubloons and pieces of gold."

Yet Clifford said could never sell the treasure he finds on board.

"It's a treasure trove of history," he said. "This is the only documented pirate treasure that's ever been discovered."

Many of the artifacts discovered on the Whydah are on display at the Whydah Pirate Shipwreck Museum in Provincetown, Mass. The exhibit also travels the world with National Geographic, Clifford said.